greybus: connection: fix offloaded-connection ping cport id

The host-device cport_ping callback expects the AP side cport id, but
was incorrectly passed the interface cport id.

Note that no host-device driver currently implements this callback, and
that it is soon even to be removed.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>